I'd recommend either contacting them directly and asking, sending a check with the "to" section left blank, or otherwise just sending a letter with some cash well-hidden. If you take the last option, make sure to send it so that an unsuspecting monk won't have to accidentally come in contact with the money itself when opening it.

Gain and loss, status and disgrace, censure and praise, pleasure and pain:these conditions among human beings are inconstant,impermanent, subject to change.

I had figured that his program was large enough that some online system had been established. This way donors could avoid bank charges and the like, thus allowing those charges to be put toward donations instead.

The sati center seems like they take online donations, they distribute the "handful of leaves", anthologies of Thanissaro's sutta translations. For thanissaro's other books, I don't know if sati center has anything to do with those.

I too am grateful to Ven. Thanissaro and would like to assist in spreading his books around. I would also like to build an extra monks' hut in the grounds of his monastery - and then, one day, reside in it!

Metta

Then the Blessed One, picking up a tiny bit of dust with the tip of his fingernail, said to the monk, "There isn't even this much form...feeling...perception...fabrications...consciousness that is constant, lasting, eternal, not subject to change, that will stay just as it is as long as eternity." (SN 22.97)